How to check if object is cycles light source?
You can do this by getting the material and checking if there is a node called Emission in the node tree, like this:

import bpy
obj = bpy.context.object
nodes = obj.data.materials[0].node_tree.nodes
if "Emission" in nodes:
    print("yes")


Likewise, you could also check for a EMISSION node by using this instead:

for node in nodes:
    if node.type == "EMISSION":
        print("yes")


That way if the user had changed the name it would still catch it.